1. Field of the Invention
This invention relates to a data storage device with a hierarchical structure corresponding to the processing speed and a method for storing data into a plurality of data storage media provided in the data storage device according to the access frequency.
2. Description of the Related Art
Recently, the need for data storage devices with a large memory capacity increases and various types of storage devices are proposed and manufactured (refer to "Advent of Two Types of File Server Dedicated Devices Used in Large-Scale Network", Electronics, 1991. 4. 15 No. 525). However, these dedicated devices are extremely expensive.
In order to realize the data storage device with large capacity at low cost, it is advantageous to realize the device by combining a portable type data storage media of large capacity and an autochanger.
However, in general, since the storage system using the autochanger requires a long time for changing the media and the access speed is low, various designs are made (Jpn. Pat. Application No. 5-238437, Jpn. Pat. Application No. 6-45486, Jpn. Pat. Application No. 6-207771).
It is also a common practice to use a data storage medium such as a cache and hierarchically store data, and various designs are proposed to serve the purpose (refer to Proceedings of the 1993 ACM SIGMOD vol. 22, No. 2, pp 297 to 306, "The LRU-K Page Replacement Algorithm For Database Disk Buffering", Jpn. Pat. Appln. KOKOKU Publication No. 5-71976).
Data storage in a hierarchical form (multistage form) is effective to enhance the access speed and the performance can be improved by data storage in a multistage form. The hierarchy is determined according to the order of a semiconductor memory (cache memory), a magnetic disk device, an optical disk loaded on the optical disk drive, and an optical disk received in the housing portions (shelves) of the autochanger and data is stored according to the access frequency in the hierarchical order.
However, it naturally becomes necessary to place storage data of each hierarchy in optimum locations since the access frequencies and the access performances required by the users are different.
In the prior art, optimization of storage data for the characteristic of each hierarchy is not always effected, and as a result, it becomes impossible to commonly provide high performance services to every data stored although the performance is high as a whole.
Various proposals have been made to improve the performance (refer to Jpn. Pat. Application No. 5-238437, Jpn. Pat. Application No. 6-45486, Jpn. Pat. Application No. 6-207771), but in this invention, an attempt is made to optimize the storage data in the hierarchies so as to further improve the performance.